By Chelsea Karnash

PHILADELPHIA (CBS) – Philadelphia’s Shake Shack will be shaking things up a bit on Thursday afternoon.

From 5:30 – 7 p.m., the Rittenhouse Square eatery is hosting a “Concrete by Design Shake Shack Tasting.” During the event, Shake Shack will offer two special “concretes,” their custom-designed frozen custard treats, for free.

Guests will have the choice of the “Nook’s Fall Look,” made with vanilla custard and a slice of graham cracker pumpkin pie from Nook Bakery; or the “La Colombe’s Espresso Session,” with chocolate custard, La Colombe Nizza espresso beans, cannoli shells and cinnamon fudge sauce.

The freebies are in celebration of the Shack’s DesignPhiladelphia’s installation from Shift Space Design, which allows anyone to create their own green roof or green space on a deck or patio. The installation is on display now through Oct. 14th.